Developers in common use for visualizing an electrostatic latent image formed on an electrophotographic photoreceptor include one-component developers comprising toner particles prepared by melt-kneading a binder resin, such as polystyrene, a styrene-butadiene copolymer or polyester, and a pigment or a dye as a colorant and grinding the mixture and two-component developers prepared by mixing toner particles with a carrier having an average particle size approximately equal to that of the toner or a greater size of up to 500 .mu.m, such as glass beads, iron powder, nickel powder or ferrite powder which may be coated with various resins.
When used as such, the above-described developers are insufficient in preservability (antiblocking properties), fluidity, developing properties, transfer properties, charging properties, and the like. It is therefore a practice usually followed to externally add additives thereto for improving these characteristics. Additives hitherto proposed include hydrophobic fine powders typically exemplified by hydrophobic silica (see JP-A-56-128956, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), silica fine particles combined with aluminum oxide powder, titanium oxide powder, etc. (see JP-A-60-238847), titanium oxide having been made hydrophobic (see JP-A-59-52255), and aluminum oxide-coated titanium oxide fine particles (see JP-A-57-79961). As a modification of titanium oxide particles, those having an anatase structure have been proposed (see JP-A-60-112052).
Addition of hydrophobic fine powders, such as hydrophobic silica, which is a frequently followed practice, improves preservability, fluidity, developing properties, and transfer properties to considerable degrees. However, use of hydrophobic fine powders in an amount enough to obtain improvements adversely affects the charging properties of a developer. To have satisfactory charging properties is to satisfy such requirements as to a quantity of charge, a rate of charging, a charge distribution, toner admixing properties, and environmental stability of charging. Of these charging properties, a rate of charging, a charge distribution, toner admixing properties, and environmental stability are adversely influenced by addition of silica, etc.
Mixing of alumina or titanium oxide fine particles with silica fine particles, which aims at improvements in charging rate, charge distribution, admixing properties, and environmental stability, is accompanied by a considerable reduction in quantity of charge. Moreover, the latitude for meeting various charging properties is very narrow, the improvements achieved are still insufficient, and, in particular, deterioration of environmental stability of chargeability results.
Where rutile or anatase titanium oxide is used as an additive, because it is hardly chargeable as such, it should be rendered hydrophobic by coating with alumina, etc. or by treating with a coupling agent. However, alumina-coated titanium oxide particles are not always prevented from agglomeration and also have insufficient dispersibility when added to a toner. Coupling agent-treated ones show some improvement in charging properties but are still insufficient.
On the other hand, where a polyester resin is used as a binder resin for toner particles, because a polyester resin itself possesses negative chargeability, it is received that negative chargeability is secured with no or little amount of a charge control agent being used. However, a polyester resin is disadvantageous in that the quantity of charge greatly varies with environmental changes between a high temperature and high humidity condition and a low temperature and low humidity condition. The disadvantage is especially pronounced when a pigment other than carbon black is used as a colorant.
While not clear, negative chargeability of a polyester resin is assumed to be attributed to the polar group thereof, a carbonyl ester group. It is accepted that the chargeability of the polar group is easily influenced by changes in temperature and humidity so that a toner containing the resin is also susceptible to the influences of the environment. Besides, even with a charge control agent added to a polyester resin, the environmental stability of chargeability cannot be improved markedly.
Hence, toners using a polyester resin as a binder have not reached satisfactory results in terms of environmental stability of chargeability.